Organic light-emitting diodes (OLEDs) have attracted considerable interest for the potential applications, as the next generation flat-panel displays and solid-state lighting sources. OLEDs are double charge injection devices, which require the simultaneous supply of both holes and electrons to the emissive layer. To realize facile and balanced charge transport, most highly efficient OLEDs tend to have multilayer device configurations, a hole transport layer (HTL), an electron transport layer (ETL) and an emissive layer (EML), some also have a hole-injection and an electron injection layer. The charge injection/transport layers are used to inject and transport holes and electrons to the EML, where the charges recombine and form the excitons. There is continuous need to develop new charge transport materials to improve device performance and lifetime.
In the case of the HTL layer, the process by which the layer is deposited is critical for its end-use application. Methods for depositing the HTL layer in small display applications involve evaporation of a small organic compound with a fine metal mask to direct the deposition. Solution processes, such as spin-coating, inkjet printing and roll-to-roll fabrication, offer an attractive alternative approach, in terms of their low-cost and large-area manufacturability, which is more amenable to commercial interests. With these findings in mind, new compositions and processes are still needed to deposit HTLs, and which satisfy these challenges, and which can be directly applied to large display applications.
Although some polymeric materials can be fabricated by a solution process, their batch-to-batch variations in solubility, molecular weight, and purity, can result in different processing properties and device performance. Since higher molecular precision of small-molecule materials can overcome the abovementioned discrepancies, the development of solution-processable, small-molecule materials, suitable for OLEDs, is highly desirable to realize this goal. One approach that appears promising is a solution process, which involves the deposition of a small molecule, followed by crosslinking or polymerization chemistry. There have been extensive efforts in this area along these lines; however these chemistries have their own shortcomings. In particular, the current technology can hardly produce an insoluble HTL film, with few to no reactive end groups, at desirable process conditions.
The benzocyclobutene (BCB) group is an example of a moiety that undergoes a thermally activated dimerization, typically at 200° C. or above, in this case, forming a dibenzocyclooctadiene ring, which is formed by scission of one of the cyclobutene C—C bonds, followed by an irreversible cycloaddition. It is been documented in the open literature that the substitution of oxygen-based donors, at the cyclobutene ring above, has a dramatic effect on the ring-opening temperature of the BCB (Dobish, J. N.; Hamilton, S. K.; Harth, E. Polymer Chemistry 2012, 3, 857-860); this phenomenon has yet to be utilized for OLEDs applications.
Benzocyclobutene (BCB) chemistries and their use in OLEDs are described in the following: US20040004433, US20080315757, US20080309229, US20100133566, US20110095278, US20110065222, US20110198573, US20110042661, JP2010062120, U.S. Pat. No. 7,893,160, US20110089411, US20070181874, US20070096082, CN102329411, US20120003790, WO2012052704, WO2012175975, WO2013007966, International Application PCT/CN14/084918 (filed Aug. 21, 2014), U.S. Prov. 62/039,935 (filed Aug. 21, 2014).
However, there remains a need for new compositions for improved hole-transporting materials, and for improved processing of the same. These needs have been met by the following invention.